Recent work in the field of olefin upgrading has resulted in a catalytic process for converting lower olefins to heavier hydrocarbons. Heavy distillate and lubricant range hydrocarbons can be synthesized over ZSM-5 type catalysts at elevated temperature and pressure to provide a product having substantially linear molecular conformations due to the ellipsoidal shape selectivity of certain medium pore catalysts.
Conversion of olefins to gasoline and/or distillate products is disclosed in U.S. Pat. Nos. 3,960,978 and 4,021,502 (Givens, Plank and Rosinski) wherein gaseous olefins in the range of ethylene to pentene, either alone or in admixture with paraffins are converted into an olefinic gasoline blending stock by contacting the olefins with a catalyst bed made up of a ZSM-5 type zeolite. Particular interest is shown in a technique developed by Garwood, et al., as disclosed in European patent application No. 83301391.5, published Sept. 29, 1983. In U.S. Pat. Nos. 4,150,062; 4,211,640 and 4,227,992 Garwood, et al., disclose the operating conditions for the Mobil Olefin to Gasoline/Distillate (MOGD) process for selective conversion of C.sub.3 + olefins to mainly aliphatic hydrocarbons.
In the process for catalytic conversion of olefins to heavier hydrocarbons by catalytic oligomerization using a medium pore, shape selective, acid, crystalline zeolite, such as ZSM-5 type catalyst, process conditions can be varied to favor the formation of hydrocarbons of varying molecular weight. At moderate temperature and relatively high pressure, the conversion conditions favor C.sub.10 + aliphatic product. Lower olefinic feedstocks containing C.sub.2 -C.sub.8 alkenes may be converted; however, the distillate mode conditions do not convert a major fraction of ethylene A typical reactive feedstock consists essentially of C.sub.3 -C.sub.6 mono-olefins, with varying amounts of nonreactive paraffins and the like being acceptable components.
U.S. Pat. Nos. 4,520,221, 4,568,786 and 4,658,079 to C. S. H. Chen, et al., incorporated herein by reference in their entirety, disclose further advances in zeolite catalyzed olefin oligomerization. These patents disclose processes for the preparation of lubricant range hydrocarbons by oligomerization of light olefins using zeolite catalyst such as ZSM-5. The oligomers so produced are essentially linear in structure and contain 90% internal olefin unsaturation. These unique olefinic oligomers are produced by surface deactivation of the ZSM-5 type catalyst by pretreatment with a surface-neutralizing base. Process conditions can be controlled to favor the recovery of near linear olefin oligomers containing six to twenty carbon atoms. Optionally, lubricant quality oligomers of higher carbon number can also be produced.
It is known that synthetic lubricating fluids of superior quality can be produced by oligomerization of 1-alkenes, particularly 1-decene. Building on that prior art resource, oligomers of 1-alkenes from C.sub.6 to C.sub.20 have been prepared, with commercially useful synthetic lubricants from 1-decene oligomerization yielding a distinctly superior lubricant product via either cationic, Ziegler or chromium catalyst known to be effective in the polymerization of 1-alkenes.
Theoretically, the oligomerization of 1-decene, for example, to lubricant oligomers in the C.sub.30 and C.sub.40 range can result in a very large number of structural isomers. Characterizing those oligomers that produce a preferred and superior synthetic lubricant meeting the specification requirements of wide-temperature fluidity while maintaining low pour point represents a prodigious challenge to the workers in the field. Brennan, Ind. Eng. Chem. Prod. Res. Dev. 1980, 19, 2-6, cites 1-decene trimer as an example of a structure compatible with structures associated with superior low temperature fluidity wherein the concentration of atoms is very close to the center of a chain of carbon atoms.
One characteristic of the molecular structure of 1-alkene oligomers that has been found to correlate very well with improved lubricant properties in commercial synthetic lubricants is the ratio of methyl to methylene groups in the oligomer. The ratio is called the branch ratio and is calculated from infra red data as discussed in "Standard Hydrocarbons of High Molecular Weight", Analytical Chemistry, Vol. 25, no. 10, p. 1466 (1953). Viscosity index has been found to increase with lower branch ratio. Oligomers prepared from 1-decene by cationic polymerization have branch ratios of greater than 0.20. Those prepared by chromium or Ziegler catalyzed oligomerization have lower branch ratios. Whether by rearrangement, isomerization or a yet to be elucidated mechanism, it is clear that in the art of 1-alkene oligomerization to produce synthetic lubricants as practiced to-date branching occurs and constrains the limits of achievable lubricant properties, particularly with respect to viscosity index Obviously, increased branching increases the number of isomers in the oligomer mixture, orienting the composition away from the structure which would be preferred from a consideration of the theoretical concepts accepted in the art.
In view of the foregoing, the practice in the synthetic lubricants field has been to oligomerize linear 1-alkene, more particularly single compounds such as 1-decene, in order to help control branching and the number of oligomeric species in the lubricant fluid. However, 1-decene and similar 1-alkenes are expensive and produce expensive lubricant fluids. Unfortunately, potentially less expensive olefins from the process of Chen, et al., are largely internal olefins and are also sightly branched, where unbranched alpha olefins are preferred. Their internal olefin structure also does not lend itself to oligomerization with either Ziegler-Natta or chromium catalysts used to produce very high quality synthetic lubricants. Cationic catalysts, e.g., BF.sub.3 or AlCl.sub.3 complexes, polymerize internal olefins but result in more branched or lower VI lubes.
Accordingly, it is an object of the present invention to provide a process for the conversion of slightly branched internal olefin oligomers, prepared from lower alkenes using surface deactivated zeolite catalyst, to alpha olefins or 1-alkenes.
Another object of the present invention is to provide a process for converting the aforementioned internal olefins to alpha olefins while retaining the low degree of branching in the internal olefin.
Yet another object of the instant invention is to prepare novel slightly branched 1-alkanol compositions from said internal olefins.
A further object of the present invention is to provide a process for production of less expensive 1-alkenes useful in the preparation of high quality polyalphaolefin (PAO) synthetic lubricant fluids.